Escalated or suppressed cocaine reward, tegmental BDNF, and accumbal dopamine caused by episodic versus continuous social stress in rats

The neural link between ostensibly aversive stress experiences and intensely rewarding drug taking remains to be delineated. Epidemiological data associate stress and the abuse of various drugs, and experimental data identify the conditions that determine how episodic social stress intensifies the motivation for cocaine and the actual self-administration of cocaine. Two types of social stress have been the focus of experimental study in Long-Evans rats, since they engender divergent changes in drug- or sugar-rewarded behavior and in neuroadaptation. Episodic social defeat stress consists of four brief confrontations between the experimental rat and an aggressive resident rat of the Long-Evans strain over the course of 10 d. Subordination stress involves the continuous exposure to an aggressive resident for 5 weeks, while living in a protective cage within the resident's home cage with brief daily confrontations. These stress experiences result in (1) increased intravenous cocaine self-administration under a fixed ratio schedule with prolonged binge-like access in episodically defeated intruder rats but suppressed cocaine intake by continuously subordinate rats; (2) deteriorated sugar preference and intake and decreased exploratory behavior in subordinate, but not intermittently defeated, rats; and (3) a sensitized dopamine (DA) response in the nucleus accumbens via in vivo microdialysis and increased tegmental brain-derived neural growth factor (BDNF) in episodically defeated rats, whereas the continuously subordinate rats show suppression of the DA and BDNF responses. These divergent neuroadaptations to social stress may represent the substrates for the intensification of cocaine "bingeing" relative to the anhedonia-like deterioration of reward processes during subordination stress.